Welcome to LLVM! In order to get started, you first need to know some basic
information.

First, LLVM comes in three pieces. The first piece is the LLVM suite. This
contains all of the tools, libraries, and header files needed to use LLVM. It
contains an assembler, disassembler, bitcode analyzer and bitcode optimizer. It
also contains basic regression tests that can be used to test the LLVM tools and
the Clang front end.

The second piece is the Clang front end. This
component compiles C, C++, Objective C, and Objective C++ code into LLVM
bitcode. Once compiled into LLVM bitcode, a program can be manipulated with the
LLVM tools from the LLVM suite.

There is a third, optional piece called Test Suite. It is a suite of programs
with a testing harness that can be used to further test LLVM’s functionality
and performance.

The LLVM Getting Started documentation may be out of date. So, the Clang
Getting Started page might also be a
good place to start.

Here’s the short story for getting up and running quickly with LLVM:

Read the documentation.

Read the documentation.

Remember that you were warned twice about reading the documentation.

In particular, the relative paths specified are important.

Checkout LLVM:

cdwhere-you-want-llvm-to-live

svncohttp://llvm.org/svn/llvm-project/llvm/trunkllvm

Checkout Clang:

cdwhere-you-want-llvm-to-live

cdllvm/tools

svncohttp://llvm.org/svn/llvm-project/cfe/trunkclang

Checkout Extra Clang Tools [Optional]:

cdwhere-you-want-llvm-to-live

cdllvm/tools/clang/tools

svncohttp://llvm.org/svn/llvm-project/clang-tools-extra/trunkextra

Checkout LLD linker [Optional]:

cdwhere-you-want-llvm-to-live

cdllvm/tools

svncohttp://llvm.org/svn/llvm-project/lld/trunklld

Checkout Polly Loop Optimizer [Optional]:

cdwhere-you-want-llvm-to-live

cdllvm/tools

svncohttp://llvm.org/svn/llvm-project/polly/trunkpolly

Checkout Compiler-RT (required to build the sanitizers) [Optional]:

cdwhere-you-want-llvm-to-live

cdllvm/projects

svncohttp://llvm.org/svn/llvm-project/compiler-rt/trunkcompiler-rt

Checkout Libomp (required for OpenMP support) [Optional]:

cdwhere-you-want-llvm-to-live

cdllvm/projects

svncohttp://llvm.org/svn/llvm-project/openmp/trunkopenmp

Checkout libcxx and libcxxabi [Optional]:

cdwhere-you-want-llvm-to-live

cdllvm/projects

svncohttp://llvm.org/svn/llvm-project/libcxx/trunklibcxx

svncohttp://llvm.org/svn/llvm-project/libcxxabi/trunklibcxxabi

Get the Test Suite Source Code [Optional]

cdwhere-you-want-llvm-to-live

cdllvm/projects

svncohttp://llvm.org/svn/llvm-project/test-suite/trunktest-suite

Configure and build LLVM and Clang:

Warning: Make sure you’ve checked out all of the source code
before trying to configure with cmake. cmake does not pickup newly
added source directories in incremental builds.

The build uses CMake. LLVM requires CMake 3.4.3 to build. It
is generally recommended to use a recent CMake, especially if you’re
generating Ninja build files. This is because the CMake project is constantly
improving the quality of the generators, and the Ninja generator gets a lot
of attention.

To use LLVM modules on Win32-based system, you may configure LLVM
with -DBUILD_SHARED_LIBS=On.

MCJIT not working well pre-v7, old JIT engine not supported any more.

Note that Debug builds require a lot of time and disk space. An LLVM-only build
will need about 1-3 GB of space. A full build of LLVM and Clang will need around
15-20 GB of disk space. The exact space requirements will vary by system. (It
is so large because of all the debugging information and the fact that the
libraries are statically linked into multiple tools).

If you are space-constrained, you can build only selected tools or only
selected targets. The Release build requires considerably less space.

The LLVM suite may compile on other platforms, but it is not guaranteed to do
so. If compilation is successful, the LLVM utilities should be able to
assemble, disassemble, analyze, and optimize LLVM bitcode. Code generation
should work as well, although the generated native code may not work on your
platform.

Compiling LLVM requires that you have several software packages installed. The
table below lists those required packages. The Package column is the usual name
for the software package that LLVM depends on. The Version column provides
“known to work” versions of the package. The Notes column describes how LLVM
uses the package and provides other details.

LLVM is very demanding of the host C++ compiler, and as such tends to expose
bugs in the compiler. We are also planning to follow improvements and
developments in the C++ language and library reasonably closely. As such, we
require a modern host C++ toolchain, both compiler and standard library, in
order to build LLVM.

For the most popular host toolchains we check for specific minimum versions in
our build systems:

Clang 3.1

GCC 4.8

Visual Studio 2015 (Update 3)

Anything older than these toolchains may work, but will require forcing the
build system with a special option and is not really a supported host platform.
Also note that older versions of these compilers have often crashed or
miscompiled LLVM.

For less widely used host toolchains such as ICC or xlC, be aware that a very
recent version may be required to support all of the C++ features used in LLVM.

We track certain versions of software that are known to fail when used as
part of the host toolchain. These even include linkers at times.

GNU ld 2.16.X. Some 2.16.X versions of the ld linker will produce very long
warning messages complaining that some “.gnu.linkonce.t.*” symbol was
defined in a discarded section. You can safely ignore these messages as they are
erroneous and the linkage is correct. These messages disappear using ld 2.17.

GNU binutils 2.17: Binutils 2.17 contains a bug which causes huge link
times (minutes instead of seconds) when building LLVM. We recommend upgrading
to a newer version (2.17.50.0.4 or later).

GNU Binutils 2.19.1 Gold: This version of Gold contained a bug which causes
intermittent failures when building LLVM with position independent code. The
symptom is an error about cyclic dependencies. We recommend upgrading to a
newer version of Gold.

This section mostly applies to Linux and older BSDs. On Mac OS X, you should
have a sufficiently modern Xcode, or you will likely need to upgrade until you
do. Windows does not have a “system compiler”, so you must install either Visual
Studio 2015 or a recent version of mingw64. FreeBSD 10.0 and newer have a modern
Clang as the system compiler.

However, some Linux distributions and some other or older BSDs sometimes have
extremely old versions of GCC. These steps attempt to help you upgrade you
compiler even on such a system. However, if at all possible, we encourage you
to use a recent version of a distribution with a modern system compiler that
meets these requirements. Note that it is tempting to install a prior
version of Clang and libc++ to be the host compiler, however libc++ was not
well tested or set up to build on Linux until relatively recently. As
a consequence, this guide suggests just using libstdc++ and a modern GCC as the
initial host in a bootstrap, and then using Clang (and potentially libc++).

The first step is to get a recent GCC toolchain installed. The most common
distribution on which users have struggled with the version requirements is
Ubuntu Precise, 12.04 LTS. For this distribution, one easy option is to install
the toolchain testing PPA and use it to install a modern GCC. There is
a really nice discussions of this on the ask ubuntu stack exchange. However,
not all users can use PPAs and there are many other distributions, so it may be
necessary (or just useful, if you’re here you are doing compiler development
after all) to build and install GCC from source. It is also quite easy to do
these days.

For more details, check out the excellent GCC wiki entry, where I got most
of this information from.

Once you have a GCC toolchain, configure your build of LLVM to use the new
toolchain for your host compiler and C++ standard library. Because the new
version of libstdc++ is not on the system library search path, you need to pass
extra linker flags so that it can be found at link time (-L) and at runtime
(-rpath). If you are using CMake, this invocation should produce working
binaries:

If you fail to set rpath, most LLVM binaries will fail on startup with a message
from the loader similar to libstdc++.so.6:version`GLIBCXX_3.4.20'notfound. This means you need to tweak the -rpath linker flag.

When you build Clang, you will need to give it access to modern C++11
standard library in order to use it as your new host in part of a bootstrap.
There are two easy ways to do this, either build (and install) libc++ along
with Clang and then use it with the -stdlib=libc++ compile and link flag,
or install Clang into the same prefix ($HOME/toolchains above) as GCC.
Clang will look within its own prefix for libstdc++ and use it if found. You
can also add an explicit prefix for Clang to look in for a GCC toolchain with
the --gcc-toolchain=/opt/my/gcc/prefix flag, passing it to both compile and
link commands when using your just-built-Clang to bootstrap.

The remainder of this guide is meant to get you up and running with LLVM and to
give you some basic information about the LLVM environment.

The later sections of this guide describe the general layout of the LLVM
source tree, a simple example using the LLVM tool chain, and links to find
more information about LLVM or to get help via e-mail.

Throughout this manual, the following names are used to denote paths specific to
the local system and working environment. These are not environment variables
you need to set but just strings used in the rest of this document below. In
any of the examples below, simply replace each of these names with the
appropriate pathname on your local system. All these paths are absolute:

SRC_ROOT

This is the top level directory of the LLVM source tree.

OBJ_ROOT

This is the top level directory of the LLVM object tree (i.e. the tree where
object files and compiled programs will be placed. It can be the same as
SRC_ROOT).

If you have the LLVM distribution, you will need to unpack it before you can
begin to compile it. LLVM is distributed as a set of two files: the LLVM suite
and the LLVM GCC front end compiled for your platform. There is an additional
test suite that is optional. Each file is a TAR archive that is compressed with
the gzip program.

This will create an ‘llvm’ directory in the current directory and fully
populate it with the LLVM source code, Makefiles, test directories, and local
copies of documentation files.

If you want to get a specific release (as opposed to the most recent revision),
you can check it out from the ‘tags’ directory (instead of ‘trunk’). The
following releases are located in the following subdirectories of the ‘tags’
directory:

Release 3.5.0 and later: RELEASE_350/final and so on

Release 2.9 through 3.4: RELEASE_29/final and so on

Release 1.1 through 2.8: RELEASE_11 and so on

Release 1.0: RELEASE_1

If you would like to get the LLVM test suite (a separate package as of 1.4), you
get it from the Subversion repository:

Git mirrors are available for a number of LLVM subprojects. These mirrors sync
automatically with each Subversion commit and contain all necessary git-svn
marks (so, you can recreate git-svn metadata locally). Note that right now
mirrors reflect only trunk for each project.

Note

On Windows, first you will want to do gitconfig--globalcore.autocrlffalse before you clone. This goes a long way toward ensuring that
line-endings will be handled correctly (the LLVM project mostly uses Linux
line-endings).

You can do the read-only Git clone of LLVM via:

% git clone https://git.llvm.org/git/llvm.git/

If you want to check out clang too, run:

%cd llvm/tools
% git clone https://git.llvm.org/git/clang.git/

If you want to check out compiler-rt (required to build the sanitizers), run:

Since the upstream repository is in Subversion, you should use gitpull--rebase instead of gitpull to avoid generating a non-linear history
in your clone. To configure gitpull to pass --rebase by default on the
master branch, run the following command:

This leaves your working directories on their master branches, so you’ll need to
checkout each working branch individually and rebase it on top of its
parent branch.

For those who wish to be able to update an llvm repo/revert patches easily using
git-svn, please look in the directory for the scripts git-svnup and
git-svnrevert.

To perform the aforementioned update steps go into your source directory and
just type git-svnup or gitsvnup and everything will just work.

If one wishes to revert a commit with git-svn, but do not want the git hash to
escape into the commit message, one can use the script git-svnrevert or
gitsvnrevert which will take in the git hash for the commit you want to
revert, look up the appropriate svn revision, and output a message where all
references to the git hash have been replaced with the svn revision.

To commit back changes via git-svn, use gitsvndcommit:

% git svn dcommit

Note that git-svn will create one SVN commit for each Git commit you have pending,
so squash and edit each commit before executing dcommit to make sure they all
conform to the coding standards and the developers’ policy.

On success, dcommit will rebase against the HEAD of SVN, so to avoid conflict,
please make sure your current branch is up-to-date (via fetch/rebase) before
proceeding.

The git-svn metadata can get out of sync after you mess around with branches and
dcommit. When that happens, gitsvndcommit stops working, complaining
about files with uncommitted changes. The fix is to rebuild the metadata:

% rm -rf .git/svn
% git svn rebase -l

Please, refer to the Git-SVN manual (mangit-svn) for more information.

While this is using SVN under the hood, it does not require any interaction from
you with git-svn.
After a few minutes, gitpull should get back the changes as they were
committed. Note that a current limitation is that git does not directly
record file rename, and thus it is propagated to SVN as a combination of
delete-add instead of a file rename.

The SVN revision of each monorepo commit can be found in the commit notes. git
does not fetch notes by default. The following commands will fetch the notes and
configure git to fetch future notes. Use gitnotesshow$commit to look up
the SVN revision of a git commit. The notes show up gitlog, and searching
the log is currently the recommended way to look up the git commit for a given
SVN revision.

Once checked out from the Subversion repository, the LLVM suite source code must
be configured before being built. This process uses CMake.
Unlinke the normal configure script, CMake
generates the build files in whatever format you request as well as various
*.inc files, and llvm/include/Config/config.h.

Variables are passed to cmake on the command line using the format
-D<variablename>=<value>. The following variables are some common options
used by people developing LLVM.

Variable

Purpose

CMAKE_C_COMPILER

Tells cmake which C compiler to use. By
default, this will be /usr/bin/cc.

CMAKE_CXX_COMPILER

Tells cmake which C++ compiler to use. By
default, this will be /usr/bin/c++.

CMAKE_BUILD_TYPE

Tells cmake what type of build you are trying
to generate files for. Valid options are Debug,
Release, RelWithDebInfo, and MinSizeRel. Default
is Debug.

CMAKE_INSTALL_PREFIX

Specifies the install directory to target when
running the install action of the build files.

LLVM_TARGETS_TO_BUILD

A semicolon delimited list controlling which
targets will be built and linked into llc. This is
equivalent to the --enable-targets option in
the configure script. The default list is defined
as LLVM_ALL_TARGETS, and can be set to include
out-of-tree targets. The default value includes:
AArch64,AMDGPU,ARM,BPF,Hexagon,Mips,MSP430,NVPTX,PowerPC,Sparc,SystemZ,X86,XCore.

LLVM_ENABLE_DOXYGEN

Build doxygen-based documentation from the source
code This is disabled by default because it is
slow and generates a lot of output.

LLVM_ENABLE_SPHINX

Build sphinx-based documentation from the source
code. This is disabled by default because it is
slow and generates a lot of output. Sphinx version
1.5 or later recommended.

LLVM_BUILD_LLVM_DYLIB

Generate libLLVM.so. This library contains a
default set of LLVM components that can be
overridden with LLVM_DYLIB_COMPONENTS. The
default contains most of LLVM and is defined in
tools/llvm-shlib/CMakelists.txt.

LLVM_OPTIMIZED_TABLEGEN

Builds a release tablegen that gets used during
the LLVM build. This can dramatically speed up
debug builds.

Unlike with autotools, with CMake your build type is defined at configuration.
If you want to change your build type, you can re-run cmake with the following
invocation:

% cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=type SRC_ROOT

Between runs, CMake preserves the values set for all options. CMake has the
following build types defined:

Debug

These builds are the default. The build system will compile the tools and
libraries unoptimized, with debugging information, and asserts enabled.

Release

For these builds, the build system will compile the tools and libraries
with optimizations enabled and not generate debug info. CMakes default
optimization level is -O3. This can be configured by setting the
CMAKE_CXX_FLAGS_RELEASE variable on the CMake command line.

RelWithDebInfo

These builds are useful when debugging. They generate optimized binaries with
debug information. CMakes default optimization level is -O2. This can be
configured by setting the CMAKE_CXX_FLAGS_RELWITHDEBINFO variable on the
CMake command line.

Once you have LLVM configured, you can build it by entering the OBJ_ROOT
directory and issuing the following command:

% make

If the build fails, please check here to see if you are using a version of
GCC that is known not to compile LLVM.

If you have multiple processors in your machine, you may wish to use some of the
parallel build options provided by GNU Make. For example, you could use the
command:

% make -j2

There are several special targets which are useful when working with the LLVM
source code:

makeclean

Removes all files generated by the build. This includes object files,
generated C/C++ files, libraries, and executables.

makeinstall

Installs LLVM header files, libraries, tools, and documentation in a hierarchy
under $PREFIX, specified with CMAKE_INSTALL_PREFIX, which
defaults to /usr/local.

makedocs-llvm-html

If configured with -DLLVM_ENABLE_SPHINX=On, this will generate a directory
at OBJ_ROOT/docs/html which contains the HTML formatted documentation.

It is possible to cross-compile LLVM itself. That is, you can create LLVM
executables and libraries to be hosted on a platform different from the platform
where they are built (a Canadian Cross build). To generate build files for
cross-compiling CMake provides a variable CMAKE_TOOLCHAIN_FILE which can
define compiler flags and variables used during the CMake test operations.

The result of such a build is executables that are not runnable on the build
host but can be executed on the target. As an example the following CMake
invocation can generate build files targeting iOS. This will work on Mac OS X
with the latest Xcode:

The LLVM build system is capable of sharing a single LLVM source tree among
several LLVM builds. Hence, it is possible to build LLVM for several different
platforms or configurations using the same source tree.

Change directory to where the LLVM object files should live:

%cd OBJ_ROOT

Run cmake:

% cmake -G "Unix Makefiles" SRC_ROOT

The LLVM build will create a structure underneath OBJ_ROOT that matches the
LLVM source tree. At each level where source files are present in the source
tree there will be a corresponding CMakeFiles directory in the OBJ_ROOT.
Underneath that directory there is another directory with a name ending in
.dir under which you’ll find object files for each source.

If you’re running on a Linux system that supports the binfmt_misc
module, and you have root access on the system, you can set your system up to
execute LLVM bitcode files directly. To do this, use commands like this (the
first command may not be required if you are already using the module):

Public header files exported from the LLVM library. The three main subdirectories:

llvm/include/llvm

All LLVM-specific header files, and subdirectories for different portions of
LLVM: Analysis, CodeGen, Target, Transforms, etc…

llvm/include/llvm/Support

Generic support libraries provided with LLVM but not necessarily specific to
LLVM. For example, some C++ STL utilities and a Command Line option processing
library store header files here.

llvm/include/llvm/Config

Header files configured by the configure script.
They wrap “standard” UNIX and C header files. Source code can include these
header files which automatically take care of the conditional #includes that
the configure script generates.

A comprehensive correctness, performance, and benchmarking test suite for LLVM.
Comes in a separate Subversion module because not every LLVM user is interested
in such a comprehensive suite. For details see the Testing Guide document.

Executables built out of the libraries
above, which form the main part of the user interface. You can always get help
for a tool by typing tool_name-help. The following is a brief introduction
to the most important tools. More detailed information is in
the Command Guide.

bugpoint

bugpoint is used to debug optimization passes or code generation backends
by narrowing down the given test case to the minimum number of passes and/or
instructions that still cause a problem, whether it is a crash or
miscompilation. See HowToSubmitABug.html for more information on using
bugpoint.

llvm-ar

The archiver produces an archive containing the given LLVM bitcode files,
optionally with an index for faster lookup.

llvm-link, not surprisingly, links multiple LLVM modules into a single
program.

lli

lli is the LLVM interpreter, which can directly execute LLVM bitcode
(although very slowly…). For architectures that support it (currently x86,
Sparc, and PowerPC), by default, lli will function as a Just-In-Time
compiler (if the functionality was compiled in), and will execute the code
much faster than the interpreter.

llc

llc is the LLVM backend compiler, which translates LLVM bitcode to a
native code assembly file.

opt

opt reads LLVM bitcode, applies a series of LLVM to LLVM transformations
(which are specified on the command line), and outputs the resultant
bitcode. ‘opt-help’ is a good way to get a list of the
program transformations available in LLVM.

opt can also run a specific analysis on an input LLVM bitcode
file and print the results. Primarily useful for debugging
analyses, or familiarizing yourself with what an analysis does.

Utilities for working with LLVM source code; some are part of the build process
because they are code generators for parts of the infrastructure.

codegen-diff

codegen-diff finds differences between code that LLC
generates and code that LLI generates. This is useful if you are
debugging one of them, assuming that the other generates correct output. For
the full user manual, run `perldoccodegen-diff'.

emacs/

Emacs and XEmacs syntax highlighting for LLVM assembly files and TableGen
description files. See the README for information on using them.

getsrcs.sh

Finds and outputs all non-generated source files,
useful if one wishes to do a lot of development across directories
and does not want to find each file. One way to use it is to run,
for example: xemacs`utils/getsources.sh` from the top of the LLVM source
tree.

llvmgrep

Performs an egrep-H-n on each source file in LLVM and
passes to it a regular expression provided on llvmgrep’s command
line. This is an efficient way of searching the source base for a
particular regular expression.

makellvm

Compiles all files in the current directory, then
compiles and links the tool that is the first argument. For example, assuming
you are in llvm/lib/Target/Sparc, if makellvm is in your
path, running makellvmllc will make a build of the current
directory, switch to directory llvm/tools/llc and build it, causing a
re-linking of LLC.

TableGen/

Contains the tool used to generate register
descriptions, instruction set descriptions, and even assemblers from common
TableGen description files.

vim/

vim syntax-highlighting for LLVM assembly files
and TableGen description files. See the README for how to use them.

This document is just an introduction on how to use LLVM to do some simple
things… there are many more interesting and complicated things that you can do
that aren’t documented here (but we’ll gladly accept a patch if you want to
write something up!). For more information about LLVM, check out: